See the Search Bar article for help removing unwanted search engines. If the arcosine software made other unwanted changes to Firefox, such as adding a toolbar or changing your home page or other settings, see Remove a toolbar that has taken over your Firefox search or home page.

P.S. This other thread may also be helpful

• https://support.mozilla.org/questions/850780 rccosine .com has taken control of firefox

could you please specify where exactly google got replaced (see screenshot). you might also want to try installing the search-reset addon...